1. Field of the Invention
The present invention relates to a signal generating apparatus and related method, and more particularly to an apparatus that generates a reference clock signal to a controller of a universal serial bus device for synchronizing a data signal received by the universal serial bus device, and a method thereof.
2. Description of the Prior Art
In a universal serial bus (USB) system, a controller in a universal serial bus device utilizes a reference clock signal with an oscillating frequency of 12 MHz to synchronize a data signal received from a host terminal, and the reference clock signal is also utilized for synthesizing a clock signal having an oscillating frequency corresponding to the clock frequency of the data signal. For example, when the data signal received by the universal serial bus device corresponds to a high speed (HS) signal, the oscillating frequency of the clock signal is required to be 480 MHz since the data transmitting rate of the high speed data signal is 480 Mb/s. Similarly, when the data signal received by the universal serial bus device corresponds to a full speed (FS) signal, the oscillating frequency of the clock signal is required to be 12 MHz, and when the data signal received by the universal serial bus device corresponds to a low speed (LS) signal, the oscillating frequency of the clock signal is required to be 1.5 MHz. Therefore, providing an accurate 12 MHz reference clock signal, such as one that has an accuracy of +/−500 ppm, for the controller is a critical issue in the universal serial bus system field.
One conventional method is to utilize a crystal oscillator to generate the 12 MHz reference clock signal for the controller, in which the crystal oscillator is externally coupled to the controller of the universal serial bus. However, since the cost of the crystal oscillator is relatively high and the crystal oscillator is hard to embed into the universal serial bus device, another conventional way is to utilize an inductor-capacitor (LC) oscillator to generate the 12 MHz reference clock signal rather the crystal oscillator. Compared to the method utilizing the crystal oscillator, the LC oscillator is easier to embed into the universal serial bus device. However, since the oscillating frequency of the reference clock signal is hard to control when the inductor and capacitor are embedded in the integrated circuit, providing a low cost and relatively simple method of generating an accurate 12 MHz reference clock signal is a significant concern in the field of universal serial bus systems.